Observing and Predicting Tides Background

Ocean Tides


Types of

Tides in
Ocean Basins

Tidal Currents


Open-Ocean Tides

Observing the changing water levels caused by astronomical tides is relatively simple and has long been important for major ports. Knowing tide levels helps pilots and ship captains avoid running aground in shallow stretches of harbor channels. In the 1850s, the port of New York began operating a real-time tide gauge that indicated to ship operators the tide level and whether it was rising or falling. Today government agencies (e.g., NOAA, Cana­dian Hydrographic Services, the British Admiralty) main­tain tide observing and prediction systems to advise mari­ners based primarily on the output of numerical models and computers. On the other hand, fewer sources of information exist on tidal currents in harbors because they are much more difficult and expensive to observe than tide levels. Whereas tide levels are nearly uniform over broad areas, tidal currents change quickly with varia­tions in winds and river discharge. Furthermore they are affected by complex shorelines and bottom topogra­phy. Nonetheless, NOAA’s Physical Oceanographic Real Time Services (PORTS) provides information on tidal cur­rents for 10 major U.S. ports.

observations and predictions of water levels, levels oscillate between 0 and 4 feet above MLLV, predictions match actual levels well
Observed (red) and predicted (blue) water level (in feet above Mean Low Water) for the tide gauge located at Nantucket Island, MA from 5 to 7 May 2004. Vertical dashed line marks the "present" time. [From NOAH, National Ocean Service, Center for Operational Oceanographic Products and Services.]

Predictions and real-time observations of tides are distributed electronically to ship operators. In the United States such predictions are made by NOAA's National Ocean Service CO-OPS (Center for Operational Oceano­graphic Products and Services) for more than 3,000 loca­tions and are available to the public online. Consider how tide predictions are made.

Periods of motions of the Earth, sun, and moon in space (i.e., orbits and rotation) are fixed and known precisely. The predictability of the movements of tide-generating celestial bodies means that astronomical tides can also be predicted with great accuracy. Tides are waves so that local tides can be resolved mathematically into their various components, called partial tides. Partial tides are forecasted individually and added together to predict the height and timing of future local tides. Although as few as four partial tides can account for 70% of the total tidal range, some 60 components are commonly used (to account for both astronomical and non-astronomical factors). More' than 100 components must be considered to predict accurately, the tides along a complex coastline, such as that of Alaska. Local astronomical tides are best predicted when based on data collected for at least 18.6 years, a period that encompasses most of the astronomical configurations of the Earth-moon-sun system that generate the tides. However, only a single year of tide gauging data usually suffices for very reasonable tide predictions. In most cases, local winds and atmospheric pressure variations are the primary causes of the difference between the actual and predicted tide.

Adapted from DataStreme Ocean and
used with permission of the
American Meteorological Society.